A radio controlled clock is a timepiece capable of adjusting its time by receiving and decoding a special time code signal. The time code signal is encoded with the current time and date and may also contain a daylight savings time and/or leap year indicator. The time code signal may also contain parity bits for ensuring accurate reception. Typically, this time code signal modulates a low frequency carrier signal which is transmitted by a government-established radio station. Several governments throughout the world have established one or more radio stations to broadcast such time code signals, including: the United States' WWVB broadcasting at 60 kHz; the United Kingdom's MSF broadcasting at 60 kHz; Germany's DCF77 broadcasting at 77.5 kHz; Japan's JJY broadcasting at both 40 kHz (transmitting in the Fukushima prefecture) and 60 kHz (transmitting on the border of the Saga prefecture and the Fukoka prefecture); China's BPC broadcasting at 68.5 kHz; Switzerland's HGB broadcasting at 75 kHz; and eastern Russia's RTZ broadcasting at 50 kHz. Additionally, some transmitters in the LORAN-C navigation system (which broadcast at 100 kHz) transmit time code signals which are synchronized to Coordinated Universal Time (UTC). Each of these radio stations modulates the carrier in substantially the same manner: reduced carrier pulse width modulation. However, since different radio stations generally broadcast time code signals on different frequencies, a radio controlled clock marketed for operation in more than one location and/or country needs to be designed to receive time code signals on multiple frequencies.
Broadcast time code signals are generated by modulating a carrier signal with a time code signal. Generally, the modulation is accomplished by the following: a carrier signal is locked to a precise oscillator (such as a cesium oscillator); a 60-bit time code containing at least the current time and date is generated with reference to a national time source (such as UTC); and the carrier power is dropped and restored at pre-determined times, depending on the modulated value of a specific time code bit.
Many radio controlled clocks contain one quartz crystal for time keeping purposes and at least one additional quartz crystal for demodulating the broadcast time code signal. The quartz crystal used for time keeping purposes is frequently divided to create a one pulse per second signal which drives a display mechanism. The frequency of the quartz crystal used for demodulating the broadcast time code signal correlates to the frequency of the particular radio station to be received.
FIG. 1 shows an example of a conventional radio controlled clock marketed for operation in a single location and/or country. A first quartz crystal 11 is coupled with an oscillator circuit 12 to provide a reference timing signal 13. Typically, this first quartz crystal 11 has a resonance frequency of 32768 Hz. The oscillator circuit 12 is further coupled to a frequency divider 20 which generates a real-time signal 21. The real-time signal 21 is used to drive a timing mechanism 30, and typically, has a frequency of one pulse per second. A low frequency broadcast time code signal 41 is received by an antenna 42 and amplified by an RF amplifier 43 to generate a modulated time code signal 44. The RF amplifier 43 is coupled to a second quartz crystal 51 to produce a time code signal 55. The resonance frequency of the second quartz crystal 51 is determined by the location and/or country in which the radio controlled clock is specified to operate. For example, a unit marketed for operation in the United States may have a second quartz crystal 51 with a resonance frequency of 60 kHz. The time code signal 55 is received by a radio receiver/time code decoder 60 which generates a time setting and/or correction signal 61. The timing mechanism 30, by receiving the time setting and/or correction signal 61, may therefore be synchronized with the broadcast time code signal 41.
More recent radio controlled clocks are marketed for operation in multiple locations and/or countries and therefore are able to receive multiple broadcast time code signals on different frequencies. FIG. 2 shows an example of how conventional multi-channel radio controlled clocks may differ from conventional single-channel radio controlled clocks. A low frequency broadcast time code signal 141 is received by an antenna 142 and amplified by an RF amplifier 143 which generates a modulated time code signal 144. A quartz crystal matrix 150 receives the modulated time code signal 144. The quartz crystal matrix may include quartz crystals 151, 152, 153 to convert the modulated time code signal 144 into the time code signal 155. For example, a radio controlled clock marketed for operation in the United States, Japan, and Germany may have one each of quartz crystals with resonance frequencies of 60 kHz, 40 kHz, and 77.5 kHz. The switching matrix 154 determines which of the plurality of quartz crystals 151, 152, 153 are electrically connected and is configured by a selectable frequency control signal 170. However, in some implementations, quartz crystals 151, 152, 153 are all electrically connected and thus the switching matrix 154 is not necessary. In such an implementation, the radio controlled clock will be used in locations where only one broadcast time code signal 141 is present, and thus, a valid time code signal 155 will be generated by only one of the quartz crystals 151, 152, 153. Similar to the conventional single channel radio controlled clock, the time code signal 155 is received by a radio receiver/time code decoder 160 to produce a time setting and/or correction signal 161. In addition to the multiple quartz crystals used in the quartz crystal matrix, the conventional multi-channel radio controlled clock might also have an additional quartz crystal to generate a real-time signal 21 as shown in FIG. 1.
Quartz crystals are used in conventional radio time clocks because they have very high frequency stability. The use of quartz crystals to generate a real-time signal leads to timepieces which keep very accurate time. The inherent stability of quartz crystals also increases the likelihood of accurate demodulation of a broadcast time code signal, since, the carrier of the broadcast time code signal is locked to that of a very stable cesium oscillator. However, the inclusion of multiple quartz crystals significantly increases the cost and size of such radio controlled clocks. A conventional quartz crystal radio controlled clock may contain up to N+1 quartz crystals, where N is the number of radio frequencies that the quartz crystal radio controlled clock is configured to receive. For example, a radio controlled clock which is marketed for use in the United States, Japan, and Germany may contain up to four quartz crystals. In addition to the increased product cost, there are engineering and manufacturing difficulties as well: multiple quartz crystals need to fit within the device. Thus, using multiple quartz crystals in a radio controlled clock may be disadvantageous because of increased material costs and engineering challenges.
Therefore, a need exists for a radio controlled clock that can receive radio signals at any of a plurality of frequencies but which enables the use of a single quartz crystal.